Show more / Show fewer performance issues

When there are a large number of results for a facet (in my case, 250+), there is a several-second delay (10 or more!) when clicking either "Show more" or "Show fewer". This patch tries to address that performance issue in a few ways:

1. Switch from using li:gt() to li.slice(), which is more performant, according to the jQuery docs:

Because :gt() is a jQuery extension and not part of the CSS specification, queries using :gt() cannot take advantage of the performance boost provided by the native DOM querySelectorAll() method. For better performance in modern browsers, use $("your-pure-css-selector").slice(index) instead. (http://api.jquery.com/gt-selector/)

2. Switch from using li:hidden to li.filter(':hidden'), which is more performant, according to the jQuery docs:

Because :hidden is a jQuery extension and not part of the CSS specification, queries using :hidden cannot take advantage of the performance boost provided by the native DOM querySelectorAll() method. To achieve the best performance when using :hidden to select elements, first select the elements using a pure CSS selector, then use .filter(":hidden"). (http://api.jquery.com/hidden-selector/)

3. Run slideUp() / slideDown() on a div that wraps all the list items to be shown / hidden, instead of running slideUp() / slideDown() on every list item. Running that animation on ~50 list items is OK, but when run on 250+ items, causes a massive slowdown.

4. Cache some references to frequently called elements.

Thanks!